The trend towards improved power efficiency in electronic devices has opened the door for energy harvesting solutions, which rely solely on readily available, but often weak sources of energy for powering applications. Energy harvesting brings new challenges for power management, requiring efficient operating characteristics in power management devices, but also in the circuits they control. Through a combination of low power characteristics and effective operating modes, a growing number of semiconductor devices can help designers achieve effective power management in energy harvesting designs. These designs include devices specifically intended for energy harvesting as well as low voltage direct current to direct current (DC-DC) converters intended for low power applications.
Energy harvesting methods have emerged in response to an increasing need for “zero-power” solutions for a growing class of embedded applications for example in automotive, security, medical, and wireless consumer electronics, where battery replacement is difficult, costly, or even hazardous. Energy harvesting designs leverage the ability of transducers to convert light, vibrations, temperature gradients, and radio frequency (RF) energy into useable voltage and/or current but at small levels. By accumulating the microWatts of energy available from ambient energy sources, energy harvesting designs can achieve substantially zero-power operation, i.e. consuming no more than that scavenged from their environment. Power management for energy harvesting applications requires minimum start-up and supply voltage, zero-power standby capability, ultra-low leakage and standby currents, and maximum efficiency while operating with small loads. Ultra-low power DC-DC converters address most or all of these concerns, ensuring stable voltage and smooth current needed for the application.
A DC-DC converter is an electronic circuit or electromechanical device, which is configured to convert a source of direct current (DC) from one voltage level to another. It is a type of electric power converter. Power levels range from very low (small batteries) to very high (high-voltage power transmission). DC-DC converters are typically used in portable electronic devices, such as mobile phones and laptop computers, which are supplied with power from batteries primarily. The electronic devices often contain several sub-circuits, each with its own voltage level requirement different from that supplied by the battery or an external supply (sometimes higher or lower than the supply voltage). Additionally, the battery voltage declines as its stored energy is drained. Switched DC-DC converters offer a method to increase voltage from a partially lowered battery voltage thereby saving space instead of using multiple batteries to accomplish the same thing. However, DC-DC converters or charge pumps, which can be considered to be DC-DC converters that use capacitors for energetic charge storage to raise or lower voltage, are limited by the voltage source from where they extract charges. If the voltage level is too low, the DC-DC current is not enough to charge the supply capacitor. The charge pumps have the same problem. The charge pumps would be limited by the power loss if the voltage level coming from the energy sources is not high enough.